1. Field of the Invention
The present invention relates to the process of electrochemically stripping coatings from airfoils.
2. Description of the Related Art
Gas turbine engines in aircraft are taken out of service at periodic intervals and regular maintenance service is performed on them. Part of the regular repair sequence for the blades and vanes (individually or together referred to hereafter as "airfoils") of these engines includes the removal and then replacement of the worn coatings from their surfaces. These coatings are usually either an aluminide coating or an MCrAlY coating. The underlying base metal of airfoils are generally made of either a nickel base alloy or a cobalt base alloy. These coatings provide airfoils with a thermal barrier to the hot corrosive environment in which airfoils operate.
In the past, these aluminide and MCrAlY coatings were removed from airfoils by soaking the parts either in nitric acid solutions (to remove aluminide coatings) or in hydrochloric acid solutions (to remove MCrAlY coatings) in high acid concentrations for up to six hours at elevated temperatures. Several disadvantages are associated with this soaking process.
This soaking process is extremely labor intensive and can produce non-uniform and unpredictable results. It can also damage or destroy airfoils if improperly carried out. Furthermore, each airfoil part requires extensive masking to protect areas sensitive to the acid soaking solution. Such areas include internal surfaces and the root section of the airfoil. These masking operations are costly, add significant time to the repair process and, if not properly carried out, can lead to damaged or destroyed parts. Still further, these soaking processes may result in extensive amounts of acidic waste solution that must be properly disposed of as well as have a long cycle time and require relatively large amounts of energy to heat the acidic solutions.
Accordingly, a better airfoil stripping process is needed by the engine maintenance and repair industry. This better airfoil stripping process should be one that has a reduced cycle time; requires a reduced amount of labor; requires less masking and lower operating temperatures; produces less hazardous waste effluent; requires less heating energy; and produces uniform and predictable stripping results so that fewer parts are damaged, destroyed or require recycling. The present invention provides a solution to these needs.